Thank you very much for the useful information Christine.
This is most likely due to bursitis (a bursa is a pad of fat that provides a cushion effect the joint structures when it moves) --- an inflammatory condition where the bursa becomes swollen and hence painful.
Another possibility is of tendonitis (inflammation of the tendons moving the knee joint).
A bony spur (outgrowth) can be another problem.
You need a comprehensive clinical examination by your doctor discussing the above mentioned possibilities.
An X-ray/MRI of the joint will be mandatory to diagnose the underlying problem with certainty, and will also help to look at the joint in detail (ruling out problems with the joint surface itself---arthritis).
Your doctor may offer you physiotherapy, pain medication, injection in to the joint --- or eventually a surgery if all the measures fail (which is little unlikely to presume at this stage).
Until you get a consultation, avoid postures that make the pain worse.
Apply ice packs to joint.
And can take Motrin or Aleve (to reduce both the pain as well as inflammation).
I would have been worried too if i were at your place but let me reassure you that this is a treatable condition and carries a good prognosis with appropriate management.
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